This invention relates to a process and apparatus for manufacturing low sulfur cement clinker which utilizes a suspension preheater, suspension calcining furnace, rotary kiln as the clinkering furnace and a clinker cooler.
In the manufacture of cement, many raw components contain sufficient quantities of sodium, potassium, chlorine, and sulfur to generate problems in the quality of the final cement clinker. Consequently, specifications for cement are usually made which define the maximum concentrations of these compounds that are allowable in the final clinker. In order to manufacture clinker according to these specifications, it becomes necessary for the manufacturer to either find raw materials which are low in these elements or find some way to remove a portion of these elements from the raw material before the final clinkering stage.
A means currently being used to remove the undesirable elements has been the gas bypass in the riser duct from the kiln feed hood. Since the compounds of sodium, potassium, chlorine and sulfur tend to violatize in the kiln prior to reaching clinkering temperatures, the fumes are picked up by the kiln exhaust gases and then transported into the kiln feed hood and riser duct. As these fumes cool, they reach a point where they condense on the surface of the incoming cement raw feed and again travel back down the kiln to the hot zone where they will again be violatized. This cycling process will continue until the concentration of the undesirable compounds in the gas stream become so high that it exceeds the partial pressure of the compound and the violatization process is retarded. The compounds begin to stay in the material being processed and appear in the cement clinker.
When the partial pressures are reached, the exhaust gases in the kiln are at their highest level of concentration of compounds of sodium, potassium, chlorine and sulfur. To remove some of these compounds, a bypass duct is usually placed on the feed hood just above the kiln exhaust. A draft is induced in the bypass duct which is greater than the draft in the riser duct thus removing a fraction of the exhaust gases laden with the undesirable compounds. In conventional processes, these exhaust gases are then precipitated onto some dust and removed from the system. Unfortunately, the bypass removal system is effective only if the compounds can be violatized at the temperatures and atmospheres normally found in a cement kiln. There are instances where this is not the case.
U.S. Pat. No. 3,923,536 describes a cement manufacturing system with preheater and precalcining equipment wherein a mixing chamber is provided to mix hot kiln exhaust gases and cooler vent gases to aid in the removal of volatile matters such as alkali, chlorine and sulfur. This system is not believed to be adequate to remove all of the sulfur compounds.
The process of U.S. Pat. No. 3,923,536 assumes that the sulfur will leave the kiln in the form of volatilized calcium sulfate, sodium sulfate and potassium sulfate. These compounds do not decompose at high temperatures and oxidizing atmospheres, but instead, they remain intact and sublime (vaporize), all the time remaining as sulfates. The patentee reports that only 10-30% of these compounds that are introduced into the system are carried out of the back end of the kiln while the remaining 70-90% are discharged in the cement clinker.
According to the present invention, fuel is added to the feed end of the kiln to produce a reducing atmosphere. In this type of environment the sulfates decompose releasing the sulfur as SO.sub.2 gas, leaving behind calcium oxide which is very stable as a solid up to termpratures of 4658 F.
According to the present invention, a means is described for gasifying practically all of the sulfur in the raw meal by operating the back end of the kiln under reducing atmosphere. At temperatures greater than approximately 1040.degree. and under mildly reducing conditions, calcium sulfate which may be contained in the raw material reduces to calcium oxide and produces sulfur dioxide. The reactions can be summarized as follows: EQU CaSO.sub.4 +CO.fwdarw.CaO+CO.sub.2 +SO.sub.2 EQU CaSO.sub.4 +H.sub.2 .fwdarw.CaO+H.sub.2 O+SO.sub.2 EQU 4 CaSO.sub.4 +CH.sub.4 .fwdarw.4 CaO+2 H.sub.2 O+CO.sub.2 +4 SO.sub.2
The technique of operating at reducing conditions has been tested in portland cement kilns and has been effective in separating sulfur from the sulfur compounds in the raw meal. The clinker from the reducing kiln has also been unusually low in sodium and potassium, indicating the possibility that a reducing atmosphere promotes greater volatilization of the alkali compounds.
It has been proposed in U.S. Pat. No. 4,173,487 to operate the rotary kiln at high back end temperatures and under reducing conditions. The technique of operating the kiln under reducing conditions is undesirable because it requires additional fuel input, most of which is wasted in the kiln exhaust gas in the form of CO and H.sub.2. A second undesirable trend has been the heavy concentration of SO.sub.2 and SO.sub.3 in the kiln exhaust which would have to be removed before emitting the gas to atmosphere. According to the present invention, a method is provided which eliminates both of these undesirable conditions.
U.S. Pat. No. 4,421,563 discloses a process and apparatus for the production of cement clinker which uses solid fuel which is gasified and uses cement raw meal to desulfurize the gasified fuel. There is no removal of sulfur from the cement raw meal in this system. The process of the present invention will serve to remove sulfur from the system, whether it is part of the fuel or part of the raw meal.